Investigate the elements of a communications system (information source, transmitter, channel, receiver, destination), analyse analogue and digital signals, and apply principles such as modulation, bandwidth, signal-to-noise ratio, attenuation and multiplexing

What this dot point is asking

NESA Telecommunications Engineering Module (HSC, one of four compulsory modules per the 2011 Engineering Studies syllabus alongside Civil Structures, Personal and Public Transport, and Aeronautical Engineering) starts with the fundamentals of communication systems. You need the standard 5-element model, the analogue / digital distinction, and the core engineering parameters (modulation, bandwidth, S/N, attenuation, multiplexing) with their practical engineering implications.

The answer

The 5-element communication system model

Every communication system can be decomposed into five elements:

1. Information source. What generates the message (a speaker's voice; a camera; a sensor; a computer).
2. Transmitter. Converts the source's signal into a form suitable for transmission. Performs modulation, amplification, encoding.
3. Channel. The physical medium that carries the signal (copper wire, coaxial cable, optical fibre, free space for radio).
4. Receiver. Inverts what the transmitter did. Performs demodulation, amplification, decoding, error correction.
5. Destination. Where the received message ends up (a speaker; a screen; a control system).

Real systems often add repeaters (amplifying and re-transmitting signals along long channels) and noise (entering at any stage but most consequentially in the channel).

Analogue vs digital signals

Analogue signals vary continuously in amplitude and time. Examples: voice on traditional telephone lines; FM radio broadcast.

Digital signals take discrete values (typically two, 0 and 1) sampled at discrete times. Examples: text messages, internet traffic, modern voice calls (after analogue-to-digital conversion).

Advantages of digital:

• Noise immunity. Digital signals can be regenerated cleanly because the receiver only needs to distinguish two states. Analogue signals degrade cumulatively.
• Compression. Digital data can be compressed; analogue cannot.
• Encryption. Digital data can be encrypted; analogue cannot easily.
• Multiplexing. Multiple digital streams can share a channel via time-division multiplexing more easily than analogue.
• Error correction. Digital data can carry redundancy bits that let receivers detect and correct errors.

Advantages of analogue (where retained):

• Bandwidth efficiency for some specific applications (FM radio remains analogue).
• Simplicity of the basic transmitter/receiver for some legacy systems.
• Continuous fidelity in principle (no quantisation error), though noise eventually dominates.

The general direction of telecommunications engineering since the 1980s has been progressive digitisation: voice telephony, broadcast TV, radio, photography, and data have all moved from analogue to digital.

Modulation

Modulation is the process of impressing the information signal onto a higher-frequency carrier wave so it can be transmitted efficiently. Demodulation at the receiver recovers the original signal.

Analogue modulation:

• AM (Amplitude Modulation). The amplitude of the carrier varies with the information signal. Used in AM radio (530-1700 kHz). Vulnerable to amplitude noise; lower fidelity than FM.
• FM (Frequency Modulation). The frequency of the carrier varies with the information signal. Used in FM radio (88-108 MHz), traditional TV audio. Less vulnerable to amplitude noise; higher fidelity.
• PM (Phase Modulation). The phase of the carrier varies with the information signal. Used in some digital systems as a base for PSK.

Digital modulation:

• ASK (Amplitude Shift Keying). Bits represented by different amplitudes.
• FSK (Frequency Shift Keying). Bits represented by different frequencies. Used in early modems.
• PSK (Phase Shift Keying). Bits represented by different phases. Used in WiFi, 4G, satellite links.
• QAM (Quadrature Amplitude Modulation). Combines amplitude and phase to carry multiple bits per symbol. Used in cable modems, digital TV, 5G.

Bandwidth

Bandwidth is the range of frequencies a signal occupies, measured in Hz. Higher-bandwidth channels can carry more information per second.

• AM radio channel: ~10 kHz bandwidth per station.
• FM radio channel: ~200 kHz bandwidth per station.
• Voice telephone channel: ~3.4 kHz bandwidth.
• Fibre optic channel: gigahertz-to-terahertz bandwidth potential.

Engineering trade-off: higher bandwidth means more data capacity but also more noise picked up and more channel occupied.

Signal-to-noise ratio (S/N)

S/N is the ratio of signal power to noise power, usually expressed in decibels (dB):

S/N (dB) = 10 log_10 (Psignal / Pnoise)

Higher S/N = cleaner reception. Below a threshold S/N, demodulation fails. Engineers design systems with margin above the threshold.

Sources of noise in a communication system: thermal noise (electronic components), interference (other transmissions on nearby frequencies), atmospheric noise (lightning, solar activity), shot noise (in semiconductor devices).

Attenuation

Attenuation is the loss of signal power as it travels through a channel, measured in dB per unit distance (e.g. dB/km for fibre optic; dB per 100m for copper).

• Copper twisted pair: ~6 dB/km at voice frequencies.
• Coaxial cable: ~10-30 dB/km depending on frequency.
• Single-mode fibre optic: ~0.2 dB/km (at 1550 nm; the reason fibre dominates long-distance).
• Free space radio: varies with frequency, atmosphere, and distance (inverse-square losses plus atmospheric absorption).

Repeaters compensate for attenuation by amplifying and re-transmitting along the link.

Multiplexing

Multiplexing allows multiple signals to share a single channel.

• Frequency Division Multiplexing (FDM). Each signal occupies a different frequency band. Examples: AM radio broadcast (many stations on different frequencies sharing the broadcast spectrum); cable TV.
• Time Division Multiplexing (TDM). Each signal occupies a different time slot. Examples: traditional digital telephony; some satellite systems.
• Code Division Multiplexing (CDM). Signals share the same frequency and time but are distinguished by different codes. Used in some mobile standards.
• Wavelength Division Multiplexing (WDM). Optical fibre version of FDM; different colours of light carry different signals. Enables single-fibre data rates beyond 1 Tbps.

Examples in context

Example 1. Australian NBN as a multiplexed system. Australia's National Broadband Network combines technologies based on distance and existing infrastructure: fibre to the premises (FTTP) where economical; fibre to the node (FTTN) with copper to the home; hybrid fibre-coaxial (HFC); fixed wireless; and satellite for remote areas. Each leg uses different modulation and multiplexing. The NBN illustrates engineering trade-offs at national scale: rural areas accept lower bandwidth (satellite) because trenching fibre across continental distances is uneconomic.

Example 2. Mobile network evolution. GSM (2G, 1990s) used TDMA at ~9.6 kbps voice and basic text. 3G added CDMA and reached ~2 Mbps. 4G introduced OFDM and reached ~100 Mbps. 5G uses massive MIMO and millimetre-wave bands to reach ~1 Gbps. Each generation shifted modulation and multiplexing to extract more bandwidth from finite spectrum. The progression illustrates how engineering parameters (modulation order, bandwidth, multiplexing scheme) drive performance.

Try this

Q1. Identify the five elements of a communication system. [2 marks]

• Cue. Information source; transmitter; channel; receiver; destination.

Q2. Distinguish between AM and FM modulation, including one advantage of each. [4 marks]

• Cue. AM = amplitude varies. Advantage: simpler / less bandwidth per station. FM = frequency varies. Advantage: less vulnerable to amplitude noise / higher fidelity.

Q3. Justify the choice of optical fibre over copper twisted pair for a long-distance telecommunications backbone. [6 marks]

• Cue. Attenuation (0.2 dB/km vs ~50 dB/km broadband copper); bandwidth (gigahertz-terahertz vs MHz); resistance to electromagnetic interference; security (harder to tap); future-proofing for data growth. Acknowledge trade-off: higher initial deployment cost.